Well if they've not reoffered the discount there's no point in paying now, you might as well appeal. The time limit for the council to respond to your representations is 56 days, they're well within that.
The rejection was probably sent by post because there is no clear law on whether a notice of rejection sent by email is legally served, at least absent prior agreement, so it's what virtually all authorities do by default.
It might be that the discount was not reoffered because you didn't make both the first and second representation within 14 days.
In the circumstances I'd suggest you go onto
https://londontribunals.org.uk/ and register an appeal with "
I rely on my formal representations" and you want and see what turns up in the evidence pack. We know they have the right photos to prove their case, but whether they'll put them in front of the adjudicator is another matter.